The invention relates to rolling bearings, especially ball bearings of the deep-groove type, used in applications for high speeds of rotation and high temperatures. The invention relates in particular to the ball retaining cage which provides the correct circumferential spacing between the balls.
Ball retaining cages for rolling bearings made of synthetic material are known, including sockets for housing the balls. The sockets are separated by claws arranged in pairs, opposite a shoulder. Such retaining cages are entirely satisfactory in many applications. However, in applications where extreme speed and acceleration conditions, coupled with high operating temperatures, are encountered, this kind of cage leads to various problems. This may be the case, for example, for bearings used in multi-plate clutches, for Formula 1-type racing cars. With the clutch engaged, the two races of the bearing turn at the same speed and in the same direction at speeds of the order of 19,000 rpm. There is then no relative movement between the races, the rolling elements and the cage. However, during gear-change sequences (clutch disengaging/engaging), in the space of approximately 40 milliseconds, the inner race of the bearing slows in relation to the outer race with a possible speed difference of 9000 rpm, then accelerates again to regain a speed of rotation equal to that of the outer race, all this at temperatures of the order of 200° C. to 250° C.
At such speeds and such temperatures, a ball retaining cage made of synthetic material tends to take on a conical shape due to the action of centrifugal force. It is said to “mushroom.” In fact, due to centrifugal force, the cage is subjected to stresses which tend to deform it, mainly in the radial direction, and give it a conical shape by increasing its diameter on the less rigid side, that is to say the open side of the ball housing sockets. This deformation phenomenon is made even worse because these cages are made of synthetic material which has a tendency to soften at high temperature.
Of course, such deformations can have consequences which are detrimental to the correct operation of the rolling bearing. In particular, in cages of known type, they lead to a risk of contact between the cage and the bore of the outer race of the bearing.
The European patent application EP-A-0,304,872 describes a ball bearing cage designed to withstand high temperatures, the balls moving in a deep-groove outer race. The cage described is made of carbon. It is in the form of a solid ring comprising sockets which emerge in areas cut out of one of the edges of the cage. The width of the cut-out areas is slightly smaller than the diameter of the balls, so that they are correctly retained inside the sockets. Such a cage is expensive and is not suitable for high speeds and high accelerations because of its inertia due to a solid structure. In addition, the use of carbon for the construction of such a cage makes it relatively fragile and sensitive to impacts.